fix(BA-6914): give the three drifting FKs one name on both install paths

Three migrations create a foreign key under a hardcoded name while the
models left it unnamed, so the metadata naming convention generated a
different one. The key is created and enforced either way -- only the name
differed, depending on whether the database came from `mgr schema oneshot`
or from replaying migrations. That is not cosmetic: each of those
downgrades drops the constraint by the hardcoded name and aborted on a
fresh install.

Converge on the hardcoded names rather than the convention ones. The
migrations' downgrades already expect them and work today on migrated
databases, so this direction needs no edit to released migrations, and it
sidesteps the convention name for prometheus_query_presets, which exceeds
PostgreSQL's 63-character limit and comes back truncated with a hash
suffix. Naming a constraint explicitly is not novel here -- see
fk_deployment_policies_endpoint.

The rename looks the current name up rather than hardcoding it, so it does
not depend on SQLAlchemy's truncation algorithm and is a no-op where the
name already matches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jopemachine

src/ai/backend/manager/models/alembic/versions/c8d1e5b73f92_reconcile_fk_constraint_name_drift.py

@@ -0,0 +1,67 @@
+"""reconcile fk constraint name drift across install paths
+
+Three migrations create a foreign key under a hardcoded name, while the models
+declared the same key without one and let the metadata naming convention
+generate it. The constraint is created and enforced either way -- only the name
+differed, depending on whether the database came from `mgr schema oneshot` or
+from replaying migrations.
+
+The models now name all three explicitly, matching what the migrations create
+and what their downgrades already drop. This migration renames the constraint on
+databases that were built from the convention.
+
+The convention name is not hardcoded here: for prometheus_query_presets it
+exceeds PostgreSQL's 63-character limit and comes back truncated with a hash
+suffix, which would tie this migration to SQLAlchemy's truncation algorithm.
+Look the current name up instead and rename only when it differs, which also
+makes this a no-op on databases that already agree.
+
+Revision ID: c8d1e5b73f92
+Revises: b2f7c1a94e30
+Create Date: 2026-07-16
+
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "c8d1e5b73f92"
+down_revision = "b2f7c1a94e30"
+# Part of: NEXT_RELEASE_VERSION
+branch_labels = None
+depends_on = None
+
+# (table, column, name the migrations create and their downgrades drop)
+_TARGETS = [
+    ("kernels", "image_id", "fk_kernels_image_id"),
+    ("login_sessions", "login_client_type_id", "fk_login_sessions_login_client_type_id"),
+    ("prometheus_query_presets", "category_id", "fk_prometheus_query_presets_category_id"),
+]
+
+
+def _rename_fk_on(conn: sa.engine.Connection, table: str, column: str, target: str) -> None:
+    inspector = sa.inspect(conn)
+    if table not in inspector.get_table_names():
+        return
+    for fk in inspector.get_foreign_keys(table):
+        if fk["constrained_columns"] != [column]:
+            continue
+        current = fk["name"]
+        if not current or current == target:
+            return
+        op.execute(f'ALTER TABLE {table} RENAME CONSTRAINT "{current}" TO "{target}"')
+        return
+
+
+def upgrade() -> None:
+    conn = op.get_bind()
+    for table, column, target in _TARGETS:
+        _rename_fk_on(conn, table, column, target)
+
+
+def downgrade() -> None:
+    # Intentionally a no-op. This migration exists to make the two install paths
+    # agree; undoing it would only put the drift back -- and the convention name
+    # it came from is not reconstructible here.
+    pass

src/ai/backend/manager/models/kernel/row.py

@@ -229,7 +229,7 @@ class KernelRow(Base):  # type: ignore[misc]
     image_id: Mapped[ImageID | None] = mapped_column(
         "image_id",
         GUID(ImageID),
-        sa.ForeignKey("images.id", ondelete="SET NULL"),
+        sa.ForeignKey("images.id", name="fk_kernels_image_id", ondelete="SET NULL"),
         nullable=True,
         index=True,
     )

src/ai/backend/manager/models/login_session/row.py

@@ -43,7 +43,11 @@ class LoginSessionRow(Base):  # type: ignore[misc]
     login_client_type_id: Mapped[uuid.UUID | None] = mapped_column(
         "login_client_type_id",
         GUID,
-        sa.ForeignKey("login_client_types.id", ondelete="SET NULL"),
+        sa.ForeignKey(
+            "login_client_types.id",
+            name="fk_login_sessions_login_client_type_id",
+            ondelete="SET NULL",
+        ),
         nullable=True,
         index=True,
     )

src/ai/backend/manager/models/prometheus_query_preset/row.py

@@ -44,7 +44,11 @@ class PrometheusQueryPresetRow(Base):  # type: ignore[misc]
     category_id: Mapped[uuid.UUID | None] = mapped_column(
         "category_id",
         GUID,
-        sa.ForeignKey("prometheus_query_preset_categories.id", ondelete="SET NULL"),
+        sa.ForeignKey(
+            "prometheus_query_preset_categories.id",
+            name="fk_prometheus_query_presets_category_id",
+            ondelete="SET NULL",
+        ),
         nullable=True,
         index=True,
     )